1. Field of the Invention
The present invention concerns a method and apparatus for use with a magnetic resonance imaging apparatus, for determining the azimuthal orientation of a medical instrument from magnetic resonance (MR) signals.
2. Description of the Prior Art
For medical procedures involving in vivo insertion of a medical instrument, for example a catheter, monitoring of the position and alignment of the instrument is frequently necessary. To make placement of the catheter tip easier, the tip generally is formed with a bend. The catheter can thereby also be inserted into branched vessels. Whether the catheter reaches into a branch or passes by the branch depends on the rotation angle of the catheter around its own axis. This rotation angle in the following is designated as the azimuthal orientation. It is desirable to be able to determine the azimuthal orientation of the curved catheter tip during the procedure in order to make targeted placement in a vessel easier.
It is generally not possible to discern the orientation of the catheter at the insertion point in the body from the orientation of the catheter tip, since the catheter is not resistant to buckling or bending over its entire length.
One possibility for determining the azimuthal orientation of the catheter tip is to monitor using continuous fluoroscopic radiography. The azimuthal orientation of the catheter tip can be directly determined by this projection method. For this purpose, it is necessary to rotate the catheter, or to move the catheter forward and back, in order to detect the orientation. With radiographic observation, however, the vessel outlets into which the catheter should be inserted generally are not visible. For this reason, the catheter manipulation and probing place high demands on the experience of the treating physician and often require long radiography times, which is disadvantageous for the patient due to the x-ray dose associated therewith.
The position and alignment of the catheter or a guide wire connected thereto can be shown in a magnetic resonance image by means of magnetic resonance measurements. For this purpose, for example, the catheter can be equipped with coils for acquisition of a magnetic resonance signal. The position of the tip can be determined from the acquired signal by comparison with signals of the other coils that are used, for example, for images. The catheter is not necessarily directly shown in the magnetic resonance image, but the position of the catheter tip is superimposed in a magnetic resonance image of the surrounding vessels. The position of the catheter so acquired in this manner can be used to track the position of the catheter tip in real time by ensuring that the measurement slice in which the tip is currently located is always currently displayed so the position of the type always remains visible in the magnetic resonance image. It has not been possible, however, to also assess or to determine the azimuthal orientation of the catheter tip in real time with this known method.
The degree of the curvature of the tip depends on the size of the particular vessel in which the catheter is currently located. In narrow vessels, the tip is straightened by the pressure of the vessel walls, which reduces its degree of curvature in comparison with larger vessels in which the curvature is not altered. The significantly curved tip in large vessels can be shown in a magnetic resonance image. The actual curvature, and therewith the azimuthal orientation, however, can be detected only with difficulty even in high-resolution magnetic resonance images. In contrast to this, in narrow vessels the azimuthal orientation cannot be detected at all in the magnetic resonance image due to the straightened tip. Therefore it cannot to be seen in advance in which direction the catheter is bent immediately upon the vessel becoming wider again, or when the catheter arrives at a branch. A continuous representation of the azimuthal orientation of the tip during the entire procedure time thus has not been possible with magnetic resonance measurements.